In order to be able to supply a continuously growing population with food and other plant-derived products, people have always been interested in improving the productivity in agriculture.
The productivity of a plant can be influenced in various different ways, e.g. by improving plant growth characteristics or by delaying leaf senescence. There are several mechanisms and pathways known which are involved in plant growth and development.
Cytokinin is a plant hormone that plays positive and negative regulatory roles in many aspects of plant growth and development. It stimulates the formation and activity of shoot meristems, is able to establish sink tissues, retards leaf senescence, inhibits root growth and branching, and it plays a role in seed germination and stress responses. Analysis of cytokinin-deficient plants has shown that cytokinin plays opposite roles in shoot and root meristems and suggests that the hormone has an essential function in quantitative control of organ growth (Mok, D. W. S. & Mok, M. C. (2001) Ann. Rev. Plant Physiol. Mol. Bio. 52, 89-118). For the model plant Arabidopsis thaliana it has been shown that the cytokinin signal is perceived by three members of the cytokinin receptor family, which are sensor histidine kinases (Inoue, T. et al. (2001) Nature 409, 1060-3; Suzuki, T. et al. (2001) Plant Cell Physiol. 42, 107-13; Yamada, H. et al. (2001) Plant Cell Physiol. 42, 1017-23.). These three cytokinin receptors, AHK2, AHK3 and CRE1/AHK4, show a high degree of sequence identity, but each has distinguishing characteristics.
Recently, a gain-of-function variant of the cytokinin receptor AHK3 has been disclosed and called ore12 (see WO 2007/108931 A1). It was shown that ore12 expression in Arabidopsis thaliana yields plants with delayed leaf senescence, whereas the overall appearance of the whole plant showed no significant difference compared to wild type plants. Although expression of ore12 may lead to plants with delayed leaf senescence and thereby to plants with improved productivity, ore12 expression had no significant effect on other plant growth characteristics. Thus, there remains a need for further improvement of plant productivity.
It is an object of the present invention to provide means and methods suitable to produce transgenic plants with improved productivity and/or growth characteristics.